Android Question

I have a Samsung Galaxy Fit S5670. My phone is telling me that the Internal memory is almost full and that I can't receive messages. I have had the same problem a few months ago and I was told to download Clean Master. I did so and it fixed it. And now it's gone bad again. I have noticed that most of the things I download go to Internal storage. My memory card is only 12% full. I tried moving some apps to the memory card but not many are allowed to be moved. What should I do?
Delete things you don't need. If you need everything you have in internal memory, get a phone that has larger internal memory. You can't keep a gallon of milk in a quart jar.